This is the 'white' sauce they give you on the side when you go to a Japanese steakhouse or get hibachi. I'd been trying to find the recipe for a long time, and it seems someone else had too, because they put up a whole website about it. The author has requested linkage instead of reposting:
Japanese White Sauce
(No ads or anything, somebody really did just make a whole site after spending a long time trying to get the recipe right.)
This is very easily adapted for lowcarb by replacing the teaspoon of sugar with a packet of splenda, or other AS of your choice. I also thin mine out with more water than the recipe calls for, and add a little lemon juice.
If you've had this before, you already know why it's awesome. If not, it's very good on grilled meats and veggies (zucchini is my favorite).
I've plugged it into fitday (without the sugar, of course), and I'm coming in well under 1 carb per tablespoon.
It's like a thinner, much less aggressive ranch dressing with a tang of sweetness to it.